Genetic Variations: Latent Effects
An individual may also harbor genetic variations that have "latent" effects. These variations, found in coding and regulatory regions, may have small effects on their own, but over time, in the context of other genetic changes, infections, or environmental exposures, they may eventually alter a person's risk for cancer. They may also explain why one person responds to a drug while another does not. These variations are very difficult to study, but populations studies are attempting to unravel their effects.
Here is part of the genome from two people who are both smokers, but only one of them gets cancer. The zoom into the chromosomes of these two men shows just a sampling of the differences in variation that are responsible for their individual cancer risk. The variations themselves do not cause cancer. They only affect each person's susceptibility to the damaging effects of tobacco smoke after exposure.